Fuel-injected internal combustion engines fueled by liquid fuels, such as gasoline and diesel, and by alcohols, in part or in whole, such as ethanol, methanol, and the like, are well known. Internal combustion engines typically produce power by controllably combusting a fuel/air mixture under compression in a combustion cylinder. For spark-ignited engines, both fuel and air first enter the cylinder where an ignition source, such as a spark plug, ignites the fuel/air charge, typically just before the piston in the cylinder reaches top-dead-center of its compression stroke. In a spark ignited engine fueled by gasoline, ignition of the fuel/air charge readily occurs under almost all ambient temperatures because of the relatively low flash point of gasoline. (The term “flash point” of a fuel is defined herein as the lowest temperature at which the fuel can form an ignitable mixture in air). However, in a spark ignited engine fueled by alcohols such as ethanol, or mixtures of ethanol and gasoline having a much higher flash point, ignition of the fuel/air charge may not occur at all under cold ambient conditions.
In many geographic areas, it is highly desirable to provide some means for enhancing the cold starting capabilities of such spark-ignited engines fueled by ethanol or other blends of alcohol. There are currently several approaches to aid cold starting of such engines. For example, some engines are equipped with an auxiliary gasoline injection system for injecting gasoline into the fuel/air charge under cold start conditions. The use of such auxiliary system adds cost to the vehicle and to the operation of the vehicle and may increase the maintenance required for the engine.
Another approach to aid cold starting of spark-ignited, alcohol fueled engines is to pre-heat the fuel before being ignited in the combustion chamber by spraying the fuel directly onto an off-spaced heat source, causing the fuel to vaporize before being ignited by the spark. Yet another method of pre-heating the fuel is to provide a heat source on the fuel injector itself proximate the injector tip to pre-heat the fuel. With either of the off-spaced heat source or the self-contained heat source, it is necessary to provide sufficient heater power and heater surface area in order to effectively transfer heat to the fuel. This may be done by applying a heater formed of an electrically resistive material, such as a thick film heater element, to the injector or the spray target. In a thick film heater element, the current flowing through the element is inversely proportional to the temperature of the element. This is known as a “positive temperature coefficient” Thus, as the temperature of the element increases, the resistance of the element also increases (and the current increases).
A fuel injector having a self contained heat source requires some form of protection that will protect the fuel injector from exposure to excessive heat should the heater overheat. In an overheat event, the heater element may severely damage the metering components of the fuel injector as well as surrounding engine components. A traditional electrical fusible link that melts and opens the circuit when current flow exceeds a set limit cannot be used to protect the injector from overheating since the current of a positive temperature coefficient heater element decreases with increasing temperature.
What is needed in the art is a thermal protection for a fuel injector heated by a positive temperature coefficient heater element.
It is a principal object of the present invention to provide a simple and inexpensive device and method for automatically disconnecting the heater element from a power source when the heater element reaches a higher than pre-selected temperature.